Microelectromechanical Systems (MEMS) sensors are widely used in applications such as automotive, inertial guidance systems, household appliances, protection systems for a variety of devices, and many other industrial, scientific, and engineering systems. Such MEMS sensors are used to sense a physical condition such as acceleration, pressure, or temperature, and to provide an electrical signal representative of the sensed physical condition. Capacitive-sensing MEMS designs are highly desirable for operation in high acceleration environments and in miniaturized devices, due to their relatively low cost.
FIG. 1 shows a top view of a prior art MEMS capacitive accelerometer 20 which is adapted to sense acceleration in an X direction 24 (that is, acceleration parallel to a major planar surface of the device). Accelerometer 20 includes a movable element 26, sometimes referred to as a proof mass or shuttle, suspended above an underlying substrate 28. Suspension anchors 30 are formed on substrate 28 and compliant members 32 interconnect movable element 26 with suspension anchors 30. Pairs of fixed fingers 34 are attached to substrate 28 by fixed finger anchors 36, and a sense finger 38 extending from movable element 26 is positioned adjacent to fixed fingers 34. Sense gaps 40 are thus formed between each side of sense fingers 38 and corresponding fixed fingers 34. In a structure of this type, when movable element 26 moves in response to acceleration in X direction 24, capacitances 42 and 44 between the moving sense fingers 38 and the fixed fingers 34 change. MEMS accelerometer 20 is provided with electronic circuitry (not shown) which converts these capacitive changes to signals representative of acceleration in X direction 24.
Many MEMS sensor applications require smaller size and low cost packaging to meet aggressive cost targets. In addition, MEMS sensor applications are calling for lower temperature coefficient of offset (TCO) specifications. TCO is a measure of how much thermal stresses effect the performance of a semiconductor device, such as a MEMS sensor. A high TCO indicates correspondingly high thermally induced stress, or a MEMS device that is very sensitive to such a stress. The packaging of MEMS sensor applications often uses materials with dissimilar coefficients of thermal expansion. Thus, an undesirably high TCO often develops during manufacture or operation. In addition, stresses can result from soldering the packaged semiconductor device onto a printed circuit board in an end application. These stresses can result in the deformation of the underlying substrate 28, referred to herein as package stress. For example, deformation of substrate 28 can result in displacements, represented by arrows 46, of suspension anchors 30 and fixed finger anchors 36. Displacements 46 induced by package stress cause changes in sense capacitances 42 and 44, thus adversely affecting capacitive accelerometer 20 output.
For the typical architecture of capacitive accelerometer 20, transducer output can be approximated by the difference between capacitance 42 and capacitance 44. Transducer output will not be affected in this architecture if displacements 46 cause both capacitances 42 and 44 to change by the same magnitude and direction. Thus, the output of MEMS sensor 20 may not be affected if the displacement of movable element 26 is substantially equivalent to the average displacement of fixed fingers 34 in the sense direction, i.e., X direction 24.
However, substrate 28 may undergo some non-linear displacement due to package stress. A component of the non-linear displacement may be non-linear displacement variation, or non-uniform stretching, across a surface of substrate 28 in X direction 24. This non-linear displacement variation is represented in FIG. 1 by dashed straight lines 48. Another component of the non-linear displacement may be non-linear in-plane deformation, or curvature, of substrate 28 in X direction 24. This in-plane “curvature” across the surface of substrate 28 is represented in FIG. 1 by dashed curved lines 50. In-plane curvature 50 represents the condition in which the top and bottom edges of capacitive accelerometer 20 are displaced by a similar amount in X direction 24, but the center region of capacitive accelerometer 20 is displaced by a different amount. Non-linear displacement variation 48 and in-plane curvature 50 result in the displacement of movable element 26 that is not substantially equivalent to the average displacement of fixed fingers 34 in the sense direction, i.e., X direction 24.
Packaging stresses can be reduced by coating the transducer die with an elastomer (sometimes referred to as a “dome coat”). However, such coatings complicate the manufacturing process and lead to an undesirably larger MEMS sensor. Furthermore, as the size of MEMS sensors decrease, a stress isolating dome coat cannot readily be used.
Thus, what is needed is a low cost, compact, single die transducer that can sense along one or more axes, that is less susceptible to thermally induced package stress gradients, and that does not require the use of a dome coat or other features designed to reduce packaging stress.